Variable flow rate valves have a valve member that cooperates with a valve seat to modulate a fluid flow, usually between a minimum value which may be zero and a maximum value. An actuator moves the valve member relative to the valve seat to vary the effective cross-sectional area of the flow. The actuator may be pneumatic, electro-magnetic, piezo-electric, mechanical etc. depending on a particular application and may be controlled to achieve a continuous or discontinuous characteristic of the valve responsive to a control signal. When the response is a linear function of the control signal, the valve is often referred to as a proportional valve.